This invention relates to a hybrid of photolithography and electron beam lithography, and in particular, to an electron beam column using high numerical aperture photocathode source illumination.
Lithography is commonly employed to produce repeatable patterns on a semiconductor substrate to form, for example, integrated circuits and flat panel displays. A conventional lithography process begins with coating a substrate with a layer of resist. An image projection system, for example, using an object reticle (i.e., xe2x80x9cmaskxe2x80x9d) or sequential scanning (i.e., xe2x80x9cdirect writexe2x80x9d), exposes selected regions of the resist with optical (light) or particle (electron) beams that change the properties of the exposed regions. Using the changed properties, the resist is developed by removing the exposed or unexposed regions (depending on the type of resist) to create a patterned resist mask suitable for further processing such as etching or oxide growth.
Currently, feature sizes of integrated circuits are continuously decreasing, requiring ever finer pattern resolutions. However, the resolution of image projection systems is limited by the spot diameter size (i.e., xe2x80x9cspot resolutionxe2x80x9d) of the beams on the target region.
One such conventional technology resulting in small spot diameters is electron beam lithography. An electron beam lithography system accelerates and focuses an intense beam of electrons to direct write precise patterns on the workpiece. However, even more precise patterns are desirable to allow a reduction in feature sizes. Therefore, what is desired is a system and method for forming patterns that have finer resolution than conventional patterns.
In accordance with the present invention, a hybrid optical/particle beam lithography (imaging) apparatus includes both a laser beam source and an electron beam column. The present electron beam column includes an optically transmissive support having an index of refraction n. The support, having a photocathode source material disposed on its (first) surface opposing the (second) surface on which the laser beam is incident, receives the laser beam such that the internal angle of the marginal rays of the laser beam is xcex8 with respect to a line normal to the support second surface. The numerical aperture (N.A.) of the beam inside the support (equal to nsin xcex8) is in one embodiment greater than one, resulting in a high resolution spot size diameter incident on the on the photocathode source material. Energy from the laser beam emits a corresponding high resolution electron beam from the photocathode source material. Electromagnetic lens component(s) in one embodiment are disposed in the electron beam column downstream from the photocathode to further demagnify the electron beam.
In one embodiment, the photocathode source material support is an optically transmissive window located at the upper part of the electron beam column. The laser beam passes through the window to impinge on the photocathode source material. In another embodiment, the photocathode source material support is on an optically transmissive substrate which is located inside the electron beam column, spaced apart from the window itself. (The window is necessary because the electron beam must be inside a vacuum, and hence the electron beam is inside a housing, typically of steel. Thus in either case, the photocathode source material is located on a support, either the window or a dedicated support substrate located inside the electron beam column housing.
Since in one embodiment the numerical aperture of the support is greater than one, the spot size diameter of the laser beam incident on the underlying photocathode source is small. A corresponding high resolution electron beam is emitted which then is further demagnified, resulting in electron beam spot sizes (diameters) of high resolutions (e.g., 100 nm or less). Thus the present hybrid of a scanning laser system and an electron beam column allows continuously decreasing minimum dimension sizes for fabrication of semiconductor circuitry.
Another benefit is improving the transmission of the electron optics, which is typically proportional to (M)2 where M is the ratio of spot size at the final image of the electron optics to spot size at the photocathode. A modest value of M allows for smaller incident optical power at the photocathode, leading to improved photocathode lifetimes, and/or system throughputs.
The photocathode source material support in one embodiment is sapphire, which has desirable high thermal conductivity, strength, and transmissivity. However, sapphire is uniaxially birefringent, presenting problems. These problems are overcome by using a particular orientation of the sapphire crystal and polarization of the laser beam, so that the c-axis of the sapphire crystal is oriented in the plane of the support and the polarization of the laser beam is at 90xc2x0 of the c-axis.
Principles of the present invention will best be understood in light of the following detailed description along with the accompanying drawings.